/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive.function.server;

import reactor.core.publisher.Mono;

/**
 * Represents a function that routes to a {@linkplain HandlerFunction handler function}.
 *
 * @param <T> the type of the {@linkplain HandlerFunction handler function} to route to
 * @author Arjen Poutsma
 * @see RouterFunctions
 * @since 5.0
 */
@FunctionalInterface
public interface RouterFunction<T extends ServerResponse> {

    /**
     * Return the {@linkplain HandlerFunction handler function} that matches the given request.
     *
     * @param request the request to route
     * @return an {@code Mono} describing the {@code HandlerFunction} that matches this request,
     * or an empty {@code Mono} if there is no match
     */
    Mono<HandlerFunction<T>> route(ServerRequest request);

    /**
     * Return a composed routing function that first invokes this function,
     * and then invokes the {@code other} function (of the same response type {@code T})
     * if this route had {@linkplain Mono#empty() no result}.
     *
     * @param other the function of type {@code T} to apply when this function has no result
     * @return a composed function that first routes with this function and then the
     * {@code other} function if this function has no result
     * @see #andOther(RouterFunction)
     */
    default RouterFunction<T> and(RouterFunction<T> other) {
        return new RouterFunctions.SameComposedRouterFunction<>(this, other);
    }

    /**
     * Return a composed routing function that first invokes this function,
     * and then invokes the {@code other} function (of a different response type) if this route had
     * {@linkplain Mono#empty() no result}.
     *
     * @param other the function to apply when this function has no result
     * @return a composed function that first routes with this function and then the
     * {@code other} function if this function has no result
     * @see #and(RouterFunction)
     */
    default RouterFunction<?> andOther(RouterFunction<?> other) {
        return new RouterFunctions.DifferentComposedRouterFunction(this, other);
    }

    /**
     * Return a composed routing function that routes to the given handler function if this
     * route does not match and the given request predicate applies. This method is a convenient
     * combination of {@link #and(RouterFunction)} and
     * {@link RouterFunctions#route(RequestPredicate, HandlerFunction)}.
     *
     * @param predicate       the predicate to test if this route does not match
     * @param handlerFunction the handler function to route to if this route does not match and
     *                        the predicate applies
     * @return a composed function that route to {@code handlerFunction} if this route does not
     * match and if {@code predicate} applies
     */
    default RouterFunction<T> andRoute(RequestPredicate predicate, HandlerFunction<T> handlerFunction) {
        return and(RouterFunctions.route(predicate, handlerFunction));
    }

    /**
     * Return a composed routing function that routes to the given router function if this
     * route does not match and the given request predicate applies. This method is a convenient
     * combination of {@link #and(RouterFunction)} and
     * {@link RouterFunctions#nest(RequestPredicate, RouterFunction)}.
     *
     * @param predicate      the predicate to test if this route does not match
     * @param routerFunction the router function to route to if this route does not match and
     *                       the predicate applies
     * @return a composed function that route to {@code routerFunction} if this route does not
     * match and if {@code predicate} applies
     */
    default RouterFunction<T> andNest(RequestPredicate predicate, RouterFunction<T> routerFunction) {
        return and(RouterFunctions.nest(predicate, routerFunction));
    }

    /**
     * Filter all {@linkplain HandlerFunction handler functions} routed by this function with the given
     * {@linkplain HandlerFilterFunction filter function}.
     *
     * @param <S>            the filter return type
     * @param filterFunction the filter to apply
     * @return the filtered routing function
     */
    default <S extends ServerResponse> RouterFunction<S> filter(HandlerFilterFunction<T, S> filterFunction) {
        return new RouterFunctions.FilteredRouterFunction<>(this, filterFunction);
    }

    /**
     * Accept the given visitor. Default implementation calls
     * {@link RouterFunctions.Visitor#unknown(RouterFunction)}; composed {@code RouterFunction}
     * implementations are expected to call {@code accept} for all components that make up this
     * router function
     *
     * @param visitor the visitor to accept
     */
    default void accept(RouterFunctions.Visitor visitor) {
        visitor.unknown(this);
    }

}
